A touch screen offers a simple, intuitive interface to a computer or other data processing device. Rather than using a keyboard to type in data, a user can transfer information through a touch screen by touching an icon or by writing or drawing on a screen. Touch screens are used in a variety of information processing applications. Transparent touch screens are particularly useful for applications such as cellphones, personal data assistants (PDAs), and handheld or laptop computers.
Various methods have been used to determine touch location, including capacitive, resistive, acoustic and infrared techniques. Touch location may also be determined by sensing the force of the touch through force sensors coupled to a touch surface. Touch screens that operate by sensing touch force have several advantages over other technologies mentioned above. First, force sensors do not require the touch surface to be composed of special materials that may inhibit optical transmission through the touch surface, as in a resistive touch sensor. Further, force sensors do not rely on a lossy electrical connection to ground, as required by a capacitive touch screen, and can be operated by a finger touch, gloved hand, fingernail or other nonconductive touch instrument. Unlike surface acoustic wave technology, force sensors are relatively immune to accumulations of dirt, dust or liquids on the touch surface. Finally, a force sensor is less likely to detect a close encounter as an actual touch, which is a common problem with infrared touch screens.
Forces detected by touch screen force sensors reflect a variety of static and dynamic factors in addition to the touch force. These factors may be considered noise sources with respect to the touch signal. Noise may be introduced through the touch screen electronics, or it may be mechanical in nature. Electrical noise may be introduced, for example, in the sensor, amplifier, data conversion or signal processing stages. Mechanical noise may arise from torsion of the touch screen, movement of the touch screen device, vibration of the touch screen, and other transient factors. In addition, the touch screen force sensors may be affected by the weight of the touch surface and preloading forces applied to the force sensors during manufacture.
The touch force changes throughout the duration of a touch. A touch in a single location produces a touch force signal that increases as the touch is applied and then decreases as the touch is removed. The touch may also be moved across the surface of the touch screen, generating a dynamic touch signal at each force sensor. Accurate determination of touch presence and location requires analysis of force signals generated by the touch force, as well as elimination of the steady state and transient noise signals from various ancillary factors affecting the touch screen at a particular time.